Nonlinear distortion measurement using composite pulse waveform

ABSTRACT

The linearity of a circuit is measured by applying a test signal waveform to the circuit under test and then determining the level of the DC component or a certain low frequency component of the waveform as altered by the circuit under test. The test signal waveform has a first value for a first predetermined time period, a second value for a second predetermined time period and a third value for a third predetermined time period, the first and third values being equal in magnitude and polarity to each other, the second value being different in magnitude and polarity from the first and third values. The area of the part of the waveform above zero axis is equal to the area below the zero axis. Once the altered waveform varies in shape from the original waveform, the area of the portions of the altered waveform above and below the zero axis will be different in accordance with the nonlinearity. This difference will cause an increase in the level of the certain low frequency component which is indicative of the linearity of the circuit under test.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved apparatus for testing thelinearity of a circuit such as audio amplifiers. The principles of thisinvention also have application to measurement of nonlinear distortionsintroduced in a signal transmission system.

2. Description of the Prior Art

In the past, to test a circuit for linearity, a sine wave test signalhas been applied to a circuit under test. The output of the circuitunder test, altered in accordance with the linearity of the circuitunder test, is applied to a notch filter for filtering out thefundamental frequency of the input test signal. As a result, anyvariation in linearity of the circuit under test will cause harmonicdistortion, i.e., higher harmonics in the output of the notch filterwhich is measured and thus provides a linearity indication.

Such an approach, generally known as the "total harmonic distortionmeasurement", is unsatisfactory to test linearity in circuits such ashigh-quality audio amplifiers in which a high degree of linearity isdesired. With distortions in the input sine waveform, the harmonicdistortion introduced in the circuit under test will vary accordinglycausing a false linearity test. These variations in harmonic distortionappear to the test circuit as a difference in linearity, which in fact,is not the case. Furthermore, the background noise originating withinthe test circuit, especially filters provided at the output side thereofis superimposed upon the test signal so as to falsify the results of themeasurements made. Because of these limitations, the measurementresolution in terms of the lowest measurable distortion is in the rangeof 0.001 to 0.01%. Another drawback of this method is its inability toprovide a direct indication of the form of nonlinear distortions. Inaddition, this total harmonic distortion measurement is time-consuming.

It has long been recognized that the total harmonic distortion methodgenerally does not give good correlation with subjective assessment ofsound quality. In an effort to provide improved subjective agreement, R.A. Belcher proposed a new technique for measuring nonlinearitydistortion, known as the double comb filter method (see "A NewDistortion Measurement" by R. A. Belcher, Wireless World, May 1978, pp.36-41). This technique uses two pseudo-random noise signals combined toprovide a test signal with anharmonic components. Based on therecognition that a pseudo-random noise signal has itself a comb-likespectrum, two comb filters are employed having different combcharacteristics, which are connected in cascade to reject the testsignal from the output from the circuit under test. The distortionsignal appearing at the output of the cascaded comb filters is measuredas an indication of the linearity of the circuit. This comb filtermethod has proved to be effective, but suffers from the disadvantagethat the circuitry required is considerably complex.

Another way to measure nonlinearity distortion is disclosed in anarticle entitled "Transient Nonlinear Distortion in Audio Equipment andMethod of Measuring Same" by Yoshimutsu Hirata ("Japanese AcousticalSociety Transaction" Vol. 1-2-16, October 1977, pp. 79-80). According tothis method the test signal employed has no DC component, comprising apositive pulse and a negative pulse, in combination, the amplitudes ofwhich are different from each other. In accordance with the teaching ofthat article, the test signal is generated by a clock-operated counterand a logic circuitry whose output is applied to a circuit under testafter digital to analog conversion. The level of the resulting DCcomponent or an increase of a certain low frequency component of thetest signal as altered by the circuit under test indicates the linearityof the circuit. This method provides distortion figures that correlatemuch better than the total harmonic distortion measurement withsubjective estimates of sound quality. However, it is desirable toprovide a straightforward and inexpensive apparatus for testing acircuit for linearity.

OBJECTS AND SUMMARY OF THE INVENTION

It is a general object of the present invention to provide astraightforward and inexpensive apparatus for testing a circuit forlinearity with a view to overcoming the deficiencies of the prior art.

It is another object of the present invention to provide an apparatusfor testing the linearity of a circuit with novel means to generate atest signal waveform and a reference signal waveform.

It is a further object of the present invention to provide an apparatusfor testing a circuit for linearity with electronic switch means tosample the positive cycle of the filtered test signal as altered by thecircuit thereby improving measurement resolution.

It is still another object of the present invention to provide anapparatus for testing a circuit for linearity with the capability ofautomatically varying the amplitude of a test signal waveform to speedup a complete test.

It is still further object of the present invention to provide anapparatus for testing a circuit for linearity with means to increase thepower of a test signal to thereby enable a determination of dynamicnonlinear distortion introduced in the circuit under test.

In accordance with the principles of the invention, these and otherobjects are accomplished by providing a test signal generator includinga pair of electronic switch means. The electronic switch means aresignal controlled between "ON" and "OFF" states to apply respectiveinput DC voltages to an adder to thereby provide a test signal waveform.The resulting test signal waveform has a first value for a firstpredetermined time period, a second value for a second predeterminedtime period and a third value for a third predetermined time period. Thefirst and third value are equal in magnitude and polarity to each other,and the second value is different in magnitude and polarity from thefirst and third values. The area of the part of the waveform above thezero axis is equal to the area below the zero axis. Means is providedfor applying the test signal waveform to a circuit under test, thecircuit under test altering the shape of the waveform in accordance withthe linearity and gain of the circuit. When this occurs, the area of theportions of the altered waveform above and below the zero axis will bedifferent in accordance with the nonlinearity. This difference will giverise to a DC component coupled with an increase of a certain lowfrequency component of the test signal waveform, either of which isindicative of the linearity of the circuit under test. In accordancewith a preferred embodiment of this invention, there is provided meansfor generating and applying a reference signal waveform to the circuitunder test. The difference between the levels of the certain lowfrequency component of the test signal waveform and the reference signalwaveform as altered by the circuit under test is determined and providesby way of a divider or a pair of logarithmic amplifiers an indication ofthe linearity of the circuit under test.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram schematically illustrating an apparatusconstructed in accordance with a preferred embodiment of the presentinvention;

FIGS. 2, 3, 4 and 5 are graphs of various waveforms useful inillustrating the principles of the present invention;

FIG. 6 is a plot of the frequency spectra of the test signal waveformbefore and after being altered by a circuit under test and a referencesignal waveform;

FIG. 7 is a block diagram showing a modification of the test signalanalyzer employed in the system of FIG. 1;

FIG. 8 is a block diagram of a further embodiment of the presentinvention;

FIG. 9 is a block diagram showing an arrangement for increasing thepower of the test signal waveform to be applied to the circuit undertest; and

FIG. 10 is a graph of various waveforms useful in illustrating theoperation of the arrangement shown in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a test signal generator 10 whichincludes a master clock generator 12, switch control circuits 14 and 16,electronic switches 18 and 20, DC voltage sources 22 and 24 and an addercircuit 26. The generator 10 provides a signal waveform 28 of FIG. 2useful in testing the linearity of an audio amplifier, by way ofexample.

Signal waveform 28 is one which maintains itself at a constant level -V₁for a time T₁, at another constant level V₂ for a time T₂ and at thesame level -V₁ for a time T₃. The transitions between the two voltagelevels -V₁ and V₂ take place at t=t₁ and t₂. The amplitude of thewaveform 28 is zero immediately before t=0 and immediately after t=t₃.The waveform may be considered to be the sum of a step voltage -V₁ whosediscontinuity occurs at t=0, a step voltage V₁ +V₂ whose discontinuityoccurs at t=t₁, a step voltage -(V₁ +V₂) whose discontinuity occurs att=t₂ and a step voltage V₁ whose discontinuity occurs at t=t₃.

It is important to note that the area of the part of the waveform abovethe zero-voltage axis equals the area which is below the zero axis andalso that |V₁ | is not equal to |V₂ |. From the former requirement itwill be appreciated that the DC component of the waveform 28 is alwayszero. It should be noted in this connection that the durations T₁ and T₃of the negative parts of the waveform could be different provided thatthe "equality" requirement of the areas above and below the zero axis issatisfied. The significance of the latter requirement, i.e., |V₁ |≠|V₂ |is that it permits detection of typical static nonlinearities usuallyencountered in audio amplifiers, such as so-called S-type nonlinearity,cross-over distortion and clipping. It should also be noted that aninverted waveform of the waveform 28 could equally be employed by thetest apparatus constructed in accordance with the present invention. Intesting audio amplifiers, the durations T₁, T₃ are preferably between0.1 and 5 m sec. while T₂ is preferably between 0.05 and 0.5 m sec. Theratio T₁ /T₂ preferably ranges from 2 to 10. In case of signaltransmission systems, the duration T₂ is preferably selected to equalapproximately the reciprocal of double the anticipated maximum frequencyof the input signal.

Master clock generator 12, switch control circuits 14 and 16, electronicswitches 18 and 20, DC voltage sources 22 and 24 and adder 26 are eachin themselves conventional components which precisely generate waveform28 to the above-mentioned requirements. Specifically, the DC voltagesource 22 includes a positive supply E which is connected to apotentiometer to develop a constant level V₁ +V₂. Likewise, the DCvoltage source 24 includes a negative supply -E which is connected toanother potentiometer to develop a constant level -V₁. The outputs ofthe DC voltage sources 22 and 24 are applied to electronic switches 18and 20, respectively, which are signal controlled between the "ON" and"OFF" states. The control signals to switches 18 and 20 are derived fromswitch control circuits 14 and 16, respectively. The master clockgenerator 12 comprises a crystal-controlled oscillator whose rectangularwave output is subjected to frequency division prior to application toswitch control circuits 14 and 16.

The electronic switches 18 and 20 are connected to the adder 26 so as tosupply it with the respective voltage outputs from the DC voltagesources 22 and 24. The operation of the electronic switches 18 and 20 inresponse to control pulses from switch control circuits 14 and 16,respectively, is indicated in lines (a), (b), (c) and (d) of FIG. 3. Theduration during which electronic switch 20 remains closed is T₁ +T₂ +T₃.The outputs of the electronic switches 18 and 20 are applied to theadder 26 for synthesis to provide the output waveform 28 as shown inline (e) of FIG. 3.

Waveform 28 is applied to an input attenuator 30 of FIG. 1 whichattenuates waveform 28 to a suitable level for testing a unit undertest, i.e., an audio amplifier 32. Unit under test 32 amplifies theattenuated waveform 28 at the output thereof. The amplified waveform 28is altered in accordance with the linearity and gain of the unit undertest.

However, as is known, the amplifier in conventional audio equipment mayhave a nonlinear distortion which varies dependent on the amplitudeand/or the time properties or frequency composition of the input signal.Should the circuit of the unit under test 32 have such distortion, it ispossible for the waveform 28 to be altered by the circuit under test 32as illustrated by waveform 34 of FIG. 4. The durations T₁, and T₂ and T₃of the respective portions of waveform 34 will be the same regardless ofthe variation in shape between waveforms 28 and 34, providing that thenonlinear distortion introduced in the circuit under test is static,i.e., dependent solely on the amplitude of the signal. However, once thewaveform 34 varies in shape from the waveform 28, then the areas of theportions of the waveform above and below the zero axis will be differentin accordance with the nonlinearity, and this difference will give riseto a DC component coupled with an increase of low frequency componentsfrom their theoretical levels. It will be appreciated that theabove-mentioned inequality of amplitude for waveform 28, that is,|V.sub. 1 |≠|V₂ | insures that nonlinear distortions, if any, cause a DCcomponent as well as an increase of the low frequency components,because when |V₁ |=|V₂ | it is possible that the positive and negativeportions of the waveform 28 are identically altered. This DC componentor increased level of certain low frequency components can be measuredas an indication of the nonlinearity of the circuit under test 32.

To accomplish this objective, the altered waveform 34 appearing at theoutput of the unit under test 32 is applied to an output attenuator 36which is a conventional circuit adapted to attenuate the amplifiedwaveform 34 to a desired level. The output of the attenuator 36 is fedto a test signal analyzer 38 which comprises a bandpass filter 40 andlinearity indicating means 42. The bandpass filter 40 removes theundesired frequencies leaving the frequency output being numericallyequal to f=1/T₀ (or m/T₀, m=2, 3, . . . ). The output of the bandpassfilter 40 is connected to the linearity indicating means 42 whichindicates the degree of nonlinearity of the circuit under test. It is tobe noted that where the DC component of the altered waveform 34 is to bemeasured in lieu of the low frequency component 1/T₀ it is necessary tocompensate the measurements for circuit drifts. In such a case, asmoothening circuit may be used in place of the bandpass filter 40, forthe purpose of deriving the DC component of the altered waveform.

Generally, in accordance with the method of this invention, thelinearity of the circuit under test is measured by separately applyingthe test signal waveform 28 and a reference signal waveform 44 of FIG. 5to the circuit and thereafter determining the difference between thelevels of a certain low frequency component of both waveforms as alteredby the circuit. The reference signal waveform 44 is generated byoperating the generator 10 of FIG. 1 in such a manner that bothelectronic switches 18 and 20 are closed simultaneously for a timeperiod equal to T₂. The frequency spectrum (Fourier transform) of thetest signal waveform 28 is represented as: ##EQU1## where T₁ =T₃.

For 2ωT₁,ωT₂ <<π, the S₁ (ω) can be approximated by taking three termsof a Fourier series expansion, as follows:

    S.sub.1 (ω)≃(ωT.sub.2).sup.2 /24 V.sub.2 T.sub.2 (γ.sup.2 +3γ+2)

where V₁ (T₁ +T₃)=V₂ T₂ ; and γ=V₂ /V₁ =2T₁ /T₂.

The frequency spectrum of the reference signal waveform 44 is given as:##EQU2##

For ωT₂ <<π, the S₀ (ω) can be approximated as follows:

    S.sub.0 (ω)≃V.sub.2 T.sub.2.

Normalizing to make S₀ (ω)=1(0 dB) results in ##EQU3##

Where it is desired to test the linearity of audio amplifiers, theangular frequency ω, the width of the positive pulse T₂, and the ratioof the amplitude of the positive and negative pulses V₂ /V₁ arepreferably:

ω=2π×250_(HZ) rad./sec.;

T₂ =1/32 m sec.; and

γ=V₂ /V₁ =3.

In this case, the normalized frequency spectrum of the test signalwaveform 28 is

    S.sub.1 (ω)=0.0020 . . . ≈0.2%(-54 dB)

It is to be understood that this value (0.2%) corresponds to atheoretical level indicative of the absence of nonlinear distortions.

A plot of the frequency spectrum S₁ (ω) of the test signal waveform 28is shown in FIG. 6. In this case, the repetition period T₀ of the testsignal waveform 26 of FIG. 3(e) is selected to equal 4 m sec. Eachfrequency component is shown in thick solid line as having a separationof 1/T₀. FIG. 6 also shows the frequency spectrum S₀ (ω) of thereference signal waveform 44.

When a circuit under test alters the applied test signal waveform 28 asmanifested by waveform 34 of FIG. 4, the "equality" condition for theareas above and below the zero axis no longer holds and the alteredwaveform shows a marked increase in the level of its low frequencycomponents as indicated in dotted lines 46 in FIG. 6. In thisembodiment, it is the 250_(HZ) frequency which is of interest. Bandpassfilter 40 is adjusted to remove the undesired frequencies leaving that250HZ frequency component.

In accordance with one embodiment of the method of this invention, thereference signal waveform 44 is generated by the signal generator 10 ofFIG. 1 for calibration. During this time, the unit under test 32 isdisconnected from the apparatus of this invention, with the output ofattenuator 30 being connected directly to the input of attenuator 36.With a series of the reference signal waveform being applied,attenuator, e.g., 36 is manually adjusted until a reading of unity(100%) is indicated by the linearity indicating means 42. Then the modeof the test signal generator 10 is switched to produce a series of thetest signal waveform 28, and the generator 10 is adjusted such that thelinearity indicating means has a reading of 0.002 (0.2%) which is equalto the theoretical value as described above. This may be accomplished bymanipulating the potentiometer of the DC voltage source 24 or the switchcontrol circuit 14 or 16 to vary the magnitude of -V₁ or the length oftime period T₁ or T₃, respectively. The next step is to connect acircuit such as an audio amplifier 32 between the attenuators 30 and 36,apply the reference signal waveform 44 to the circuit under test andadjust the attenuator 36 so that the linearity indicating means 42 has areading of unity (100%). With the circuit under test 32 connected, thereading of the linearity indicating means 42 is taken by applying thetest signal waveform 28 to the circuit under test. The differencebetween the reading of the linearity indicating means and thetheoretical value gives a measure of the linearity of the circuit undertest, or a distortion figure.

Tests with the circuit 32 connected to the apparatus of FIG. 1 proceedwith varying amplitudes of the reference signal waveform 44 and the testsignal waveform 28. Attenuator 30 is manually adjusted so as to providefor variations in the amplitude of the reference and test signalwaveforms. It will be appreciated that by studying changes in thedistortion figure for varying amplitudes of the reference and testsignal waveforms it is possible to determine the form of nonlineardistortion, that is, whether it is S-type nonlinearity, crossoverdistortion or clipping.

FIG. 7 illustrates a modification of the test signal analyzer 38 shownin FIG. 1. An electronic switch 50 has been introduced to sample onlythe positive portion of the bandpass filter 40's output so as to provideincreased measurement resolution. This will best be understood fromlines (f), (g) and (h) of FIG. 3. The electronic switch 50 is suppliedwith a control signal as indicated in (g) of FIG. 3 from a switchcontrol circuit 52 which is similar to circuits 14 and 16 of FIG. 1. Theswitch control circuit 52 in turn receives a clock signal (not shown)from the master clock generator 12 through a pulse shift circuit 54, theclock signal having been delayed a suitable amount by the phase shifter54 so that the time during which the switch control 52's output is at ahigher level coincides with the positive half cycle of the bandpassfilter output. The pulse shifter 54 is manually adjusted so as tomaximize the output of the electronic switch 50. The resultantelectronic switch output as indicated in line (h) of FIG. 3 is low-passfiltered by a filter 56 to produce an output signal which is applied tothe linearity indicating means 42.

FIG. 8 shows a block diagram of an apparatus for testing a circuit forlinearity in accordance with a further embodiment of the presentinvention. This embodiment differs from that of FIG. 1 in providing thecapability of automating a series of distortion measurements to minimizethe measurement time. A ramp voltage generator 70 is employed whichgenerates at its output a ramp voltage E(t). The rate at which the rampvoltage E(t) varies is, for example, 5 volts per sec. The ramp voltageoutput is applied across a potentiometer which consists of variableresistors R₁ and R₂. The values of the variable resistors R₁ and R₂ areadjusted such that assuming the voltage at a connection between R₁ andR₂ to be zero the ratio of the voltages across R₁ and R₂ is V₂ -V₁. Thevoltages across R₁ and R₂ are applied to respective electronic switches72, similar to those 18 and 20 of FIG. 1, which are signal controlled bya switch control circuit 74. The operation of the electronic switches 72is identical to that of the electronic switches 18 and 20 and theresultant switched signals from the electronic switches 72 are suppliedto an adder 76. The output of the adder 76 is high-pass filtered orpassed through a capacitor 78 for DC cut-off and then is supplied to abuffer amplifier 80. The output of the buffer amplifier 80 is appliedthrough an attenuator 82 to a circuit under test 84.

The altered output from the circuit under test, that is, the voltageacross its 8 ohm resistance load or loudspeaker load, is supplied to atest signal analyzer 86 which is somewhat different from that of FIG. 1.As seen, a bandpass filter 88 is connected to the output of anattenuator 90 to pass only the frequency component 1/T₀ or m/T₀ (m 2,3,4. . . ) of the attenuated altered test signal waveform. The resultantfiltered signal from the lowpass filter 88 is supplied to a rectifier 92and then is applied to one input of a divider circuit 94.

The output from the attenuator 90 is also applied to a half-waverectifier 96 to sample the positive portion of each altered test signalwaveform to produce an output signal corresponding to the alteredreference signal waveform. The resultant half-wave rectified signal fromthe rectifier 96 is passed through a bandpass filter 98 having identicalcharacteristics to the filter 88 to single out the desired low frequencycomponent. The output from the bandpass filter 98 is applied to theother input of the divider 94 after rectification by a rectifier 100.The output from the divider 94 is representative of the ratio of thelevel of the certain low frequency component of the test signal waveformto that of the reference signal waveform as altered by the circuit undertest. A lowpass filter 102 is connected to the output of the half-waverectifier 96 to provide an output signal corresponding to the level ofthe certain low frequency of the positive portion of the altered testsignal waveform. The output signal of the lowpass filter 102 is appliedto the attenuator 90 through line 104 to vary the amount of attenuationprovided thereby to a suitable level. The output of the divider 94 andthat of the lowpass filter 102 are applied to the Y-axis and X-axisterminals respectively of an X-Y recorder (not shown) to form a displayof the amount of nonlinear distortion, i.e., the distortion figure,varying with the amplitude of the input test signal waveform.

It will be appreciated that if the unit under test is linear the outputof the divider 94 remains constant irrespective of changes in theamplitude of the input test signal waveform. When the divider outputvaries otherwise, it is possible to determine the form of nonlineardistortion based on the curve of the divider output. It will be alsoappreciated that a pair of logarithmic amplifiers could be employed inplace of the divider 94 to provide an indication (in dB) of thelinearity of the circuit under test.

FIG. 9 illustrates an arrangement for superimposing a rectangular pulsewaveform on the test signal waveform 28 of FIG. 2 to permit testing ofthe circuit under test 32 under increased drive conditions. Therectangular pulse waveform 110 is indicated in line (a) of FIG. 10 andis generated by an electronic switch 112 operative in response to acontrol signal from a switch control circuit 114. The switch controlcircuit 114 is similar to those indicated at 14 and 16 of FIG. 1 andresponds to the clock signal from the master clock generator 12 bygenerating the control signal. The electronic switch 112 has an inputconnected to a DC voltage source 116 and an output connected to ahighpass filter 118 consisting of a capacitor and a grounded resistor.The output of the highpass filter 118 is applied to the adder 26 of FIG.1 together with the outputs from the electronic switches 18 and 20 toprovide a composite signal waveform as indicated in line (c) of FIG. 10.The switch control 114 is adjusted such that each test signal waveform28 coincides with the positive or negative cycle of the rectangularsignal waveform 110. Also, it is important to select the repetitionperiods T₃ and T₀ of the rectangular signal waveform and the test signalwaveform respectively such that 1/T₀ <<1/T₃ to thereby isolate theparticular low frequency component of the test signal waveform whoselevel is to be measured, from the frequency components of therectangular signal waveform. With this arrangement, it is possible toincrease the power of the input signal supplied to the circuit undertest 32 to permit a determination of the presence or absence of dynamicnonlinear distortion which is introduced in the circuit dependent notonly on the amplitude but also on the time properties or frequencycomposition of the signal. Let it be assumed that a rectangular signalwaveform having an amplitude equal to half the peak value V₂ of the testsignal waveform 28 is superimposed on the test signal waveform with T₂equal to 1/32 m sec. and the repetition period T₀ equal to 4 m sec. Insuch a case, the power of the composite test signal is increased by afactor of approximately 3×10⁴.

It will be understood and appreciated that many changes can be made inthe preferred embodiments described herein and that, further, alternatemeans of implementation thereof are possible and within the skill ofthose familiar with the art. Consequently, while the present inventionhas been described by way of specific examples, it is not to be solelylimited thereto, except as defined by the appended claims.

I claim:
 1. An apparatus for measuring the linearity of a circuit,comprising:means for generating a test signal waveform having a firstvalue for a first predetermined time period, a second value for a secondpredetermined time period immediately following said first predeterminedtime period and a third value for a third predetermined time periodimmediately following said second predetermined time period, said firstand third values being equal in magnitude and polarity to each other,said second value being different in magnitude and polarity from saidfirst and third values, the area of the part of said test signalwaveform above the zero axis being equal to the area below the zeroaxis; means for applying said test signal waveform to said circuit, saidcircuit tending to alter the applied waveform in accordance with thelinearity and gain of said circuit to produce as an output signal analtered test signal waveform; and means for determining the level of apredetermined low frequency component of said altered test signalwaveform and for comparing the determined level to its theoretical levelto produce an output signal indicative of the linearity of said circuit,said theoretical level being equal to the level of said predeterminedlow frequency component of said unaltered test signal waveformmultiplied by the gain of said circuit.
 2. The apparatus of claim 1wherein said means for generating a test signal waveform comprises meansproviding a first DC voltage of said first value, means providing asecond DC voltage of said second value minus said first value, addermeans having at least a first and a second input, first switch meansoperative in response to a first control signal to couple said first DCvoltage to said first input of said adder, second switch means operativein response to a second control signal to couple said second DC voltageto said second input of said adder, means for providing said firstcontrol signal to said first switch means during said first, second andthird predetermined time periods, and means for providing said secondcontrol signal to said second switch means during said secondpredetermined time period.
 3. The apparatus of claim 1 furthercomprising means for varying the amplitude of said test signal waveformbefore being applied to said circuit.
 4. The apparatus of claim 1wherein said means for determining the level of a predetermined lowfrequency component comprises a first bandpass filter connected toreceive said altered test signal waveform and to pass therethrough onlythe predetermined low frequency component thereof, and linearityindicating means responsive to the output of said bandpass filter forproviding a signal indicating the level of said predetermined lowfrequency component of said altered test signal waveform.
 5. Theapparatus of claim 1 wherein said means for determining the level of apredetermined low frequency component comprises a first bandpass filterconnected to receive said altered test signal waveform and to passtherethrough only the predetermined low frequency component thereof,third switch means operative in response to a third control signal tocouple the output of said first bandpass filter to a lowpass filter,linearity indicating means responsive to the output of said lowpassfilter for providing a signal indicating the level of said predeterminedlow frequency component of said altered test signal waveform, and meansfor providing said third control signal to said third switch meansduring the positive cycle of the output of said bandpass filter.
 6. Theapparatus of claim 2 further comprising means providing a third DCvoltage of a fourth value, fourth switch means operative in response toa fourth control signal to couple said third DC voltage to a third inputof said adder means, DC cutoff means connected between said fourthswitch means and the third input of said adder means, means forproviding said fourth control signal to said fourth switch means at apredetermined repetition frequency to produce a rectangular waveform atthe output of said DC cutoff means, said predetermined repetitionfrequency of said rectangular waveform being substantially higher thanthe repetition frequency of said test signal waveform, the relativephase of said rectangular waveform and said test signal waveform beingsuch that each test signal waveform coincides with one of the positiveand negative cycles of said rectangular waveform.
 7. The apparatus ofclaim 1 wherein said circuit forms part of audio equipment such as anaudio amplifier and wherein said first and third predetermined timeperiods are in the range of 0.1 to 5 m sec. and said secondpredetermined time period is in the range of 0.05 to 0.5 m sec., theratio of said first predetermined time period to said secondpredetermined time period being in the range of 2 to
 10. 8. Theapparatus of claim 1 wherein said circuit forms part of a signaltransmission system and wherein said second predetermined time period isapproximately equal to the reciprocal of double the anticipated maximumfrequency of an input signal to said signal transmission system.
 9. Theapparatus of claim 1 wherein said means for generating a test signalwaveform comprises means providing a first DC voltage of said firstvalue, means providing a second DC voltage of said second value minussaid first value, adder means having at least a first and a secondinput, first switch means operative in response to a first controlsignal to couple said first DC voltage to said first input of saidadder, second switch means operative in response to a second controlsignal to couple said second DC voltage to said second input of saidadder, means for providing said first control signal to said firstswitch means during said first, second and third predetermined timeperiods, and means for providing said second control signal to saidsecond switch means during said second predetermined time period. 10.The apparatus of claim 1 further comprising means providing a third DCvoltage of a fourth value, fourth switch means operative in response toa fourth control signal to couple said third DC voltage to a third inputof said adder means, DC cutoff means connected between said fourthswitch means and the third input of said adder means, means forproviding said fourth control signal to said fourth switch means at apredetermined repetition frequency to produce a rectangular waveform atthe output of said DC cutoff means, said predetermined repetitionfrequency of said rectangular waveform being substantially higher thanthe repetition frequency of said test signal waveform, the relativephase of said rectangular waveform and said test signal waveform beingsuch that each test signal waveform coincides with one of the positiveand negative cycles of said rectangular waveform.
 11. The apparatus ofclaim 1 wherein said means for generating a test signal waveformcomprises means providing a first DC voltage of said first value, meansproviding a second DC voltage of said second value minus said firstvalue, adder means having at least a first and a second input, firstswitch means operative in response to a first control signal to couplesaid first DC voltage to said first input of said adder, second switchmeans operative in response to a second control signal to couple saidsecond DC voltage to said second input of said adder, means forproviding said first control signal to said first switch means duringsaid first, second and third predetermined time periods, and means forproviding said second control signal to said second switch means duringsaid second predetermined time period, and wherein said means forgenerating a reference signal waveform comprises means for providingsaid first and second control signals during said fourth predeterminedtime period.
 12. The apparatus of claim 11 further comprising meansproviding a third DC voltage of a fifth value, fourth switch meansoperative in response to a fourth control signal to couple said third DCvoltage to a third input of said adder means, DC cutoff means connectedbetween said fourth switch means and the third input of said addermeans, means for providing said fourth control signal to said fourthswitch means at a predetermined repetition frequency to produce arectangular waveform at the output of said DC cutoff means, saidpredetermined repetition frequency of said rectangular waveform beingsubstantially higher than the repetition frequency of said test signalwaveform, the relative phase of said rectangular waveform and said testsignal waveform being such that each test signal waveform coincides withone of the positive and negative cycles of said rectangular waveform.13. The apparatus of claim 1 wherein said means for generating a testsignal waveform comprises means providing a first DC voltage of saidfirst value, means providing a second DC voltage of said value minussaid first value, adder means having at least a first and a secondinput, first switch means operative in response to a first controlsignal to couple said first DC voltage to said first input of saidadder, second switch means operative in response to a second controlsignal to couple said second DC voltage to said second input of saidadder, means for providing said first control signal to to said firstswitch means during said first, second and third predetermined timeperiods, and means for providing said second control signal to saidsecond switch means during said second predetermined time period, andwherein said means for generating a reference signal waveform comprisesmeans for providing said first and second control signals during saidfourth predetermined time period.
 14. The apparatus of claim 1 furthercomprising means providing a third DC voltage of a fifth value, fourthswitch means operative in response to a fourth control signal to couplesaid third DC voltage to a third input of said adder means, DC cutoffmeans connected between said fourth switch means and the third input ofsaid adder means, means for providing said fourth control signal to saidfourth switch means at a predetermined repetition frequency to produce arectangular waveform at the output of said DC cutoff means, saidpredetermined repetition frequency of said rectangular waveform beingsubstantially higher than the repetition frequency of said test signalwaveform, the relative phase of said rectangular waveform and said testsignal waveform being such that each test signal waveform coincides withone of the positive and negative cycles of said rectangular waveform.15. An apparatus for measuring the linearity of a circuit,comprising:means for generating a test signal waveform having a firstvalue for a first predetermined time period, a second value for a secondpredetermined time period immediately following said first predeterminedtime period and a third value for a third predetermined time periodimmediately following said second predetermined time period, said firstand third values being equal in magnitude and polarity to each other,said second value being different in magnitude and polarity from saidfirst and third values, the area of the part of said test signalwaveform above the zero axis being equal to the area below the zeroaxis; means for applying said test signal waveform to said circuit, saidcircuit tending to alter the applied waveform in accordance with thelinearity and gain of said circuit to produce as an output signal analtered test signal waveform; and means for determining the level of aDC component of said altered test signal waveform to produce an outputsignal indicative of the linearity of said circuit.
 16. An apparatus formeasuring the linearity of a circuit, comprising:means for generating atest signal waveform having a first value for a first predetermined timeperiod, a second value for a second predetermined time periodimmediately following said first predetermined time period and a thirdvalue for a third predetermined time period immediately following saidsecond predetermined time period, said first and third values beingequal in magnitude and polarity to each other, said second value beingdifferent in magnitude and polarity from said first and third values,the area of the part of said test signal waveform above the zero axisbeing equal to the area below the zero axis; means for applying saidtest signal waveform to said circuit, said circuit tending to alter theapplied waveform in accordance with the linearity and gain of saidcircuit to produce as an output signal an altered test signal waveform;means for generating a reference signal waveform having a fourth valuefor a fourth predetermined time period, said fourth value being equal inmagnitude and polarity to said second value, said fourth predeterminedtime period being equal to said second predetermined time period; meansfor applying said reference signal waveform to said circuit, saidcircuit tending to alter the applied reference signal waveform inaccordance with the linearity of and gain of said circuit to produce asan output signal an altered reference signal waveform; and means fordetermining the respective levels of a predetermined low frequencycomponent of said altered test signal waveform and of said alteredreference signal waveform and for comparing the determined levels toeach other to produce an output signal indicative of the linearity ofsaid circuit.
 17. The apparatus of claim 12 further comprising means forvarying the amplitude of said test signal waveform and said referencesignal waveform before being applied to said circuit.
 18. The apparatusof claim 16 wherein said means for determining the respective levels ofa predetermined low frequency component comprises a first bandpassfilter connected to receive said altered test signal waveform and saidreference signal waveform and to pass therethrough only thepredetermined low frequency component thereof, and linearity indicatingmeans responsive to the output of said bandpass filter for providingsignals indicating the respective levels of said predetermined lowfrequency component of said altered test signal waveform and of saidaltered reference signal waveform.
 19. The apparatus of claim 16 whereinsaid means for determining the respective levels of a predetermined lowfrequency component comprises a first bandpass filter connected toreceive said altered test signal waveform and said reference signalwaveform and to pass therethrough only the predetermined low frequencycomponent thereof, third switch means operative in response to a thirdcontrol signal to couple the output of said first bandpass filter to alowpass filter, linearity indicating means responsive to the output ofsaid lowpass filter for providing signals indicating the respectivelevels of said predetermined low frequency component of said alteredtest signal waveform and of said reference signal waveform, and meansfor providing said third control signal to said third switch meansduring the positive cycle of the output of said bandpass filter.
 20. Theapparatus of claim 16 wherein said circuit forms part of audio equipmentsuch as an audio amplifier and wherein said first and thirdpredetermined time periods are in the range of 0.1 to 5 m sec. and saidsecond predetermined time period is in the range of 0.05 to 0.5 m sec.,the ratio of said first predetermined time period to said secondpredetermined time period being in the range of 2 to
 10. 21. Theapparatus of claim 16 wherein said circuit forms part of a signaltransmission system and wherein said second predetermined time period isapproximately equal to the reciprocal of double the anticipated maximumfrequency of an input signal to said signal transmission system.
 22. Anapparatus for measuring the linearity of a circuit, comprising:means forgenerating a test signal waveform having a first value for a firstpredetermined time period, a second value for a second predeterminedtime period immediately following said first predetermined time periodand a third value for a third predetermined time period immediatelyfollowing said second predetermined time period, said first and thirdvalues being equal in magnitude and polarity to each other, said secondvalue being different in magnitude and polarity from said first andthird values, the area of the part of said test signal waveform abovethe zero axis being equal to the area below the zero axis; means forapplying said test signal waveform to said circuit, said circuit tendingto alter the applied waveform in accordance with the linearity and gainof said circuit to produce as an output signal an altered test signalwaveform; means for generating a reference signal waveform having afourth value for a fourth predetermined time period, said fourth valuebeing equal in magnitude and polarity to said second value, said fourthpredetermined time period being equal to said second predetermined timeperiod; means for applying said reference signal waveform to saidcircuit, said circuit tending to alter the applied reference signalwaveform in accordance with the linearity of and gain of said circuit toproduce as an output signal an altered reference signal waveform; andmeans for determining the respective levels of a DC component of saidaltered test signal waveform and of said altered reference signalwaveform and for comparing the determined levels to each other toproduce an output signal indicative of the linearity of said circuit.23. An apparatus for measuring the linearity of a circuit,comprising:means for generating a test signal waveform having a firstvalue for a first predetermined time period, a second value for a secondpredetermined time period immediately following said first predeterminedtime period and a third value for a third predetermined time periodimmediately following said second predetermined time period, said firstand third values being equal in magnitude and polarity to each other,said second value being different in magnitude and polarity from saidfirst and third values, the area of the part of said test signalwaveform above the zero axis being equal to the area below the zeroaxis; means for applying said test signal waveform to said circuit, saidcircuit tending to alter the applied waveform in accordance with thelinearity and gain of said circuit to produce as an output signal analtered test signal waveform; a first bandpass filter connected toreceive said altered test signal waveform and to pass therethrough onlythe predetermined low frequency component thereof; a first rectifierconnected to receive the output of said first bandpass filter; ahalf-wave rectifier connected to receive said altered test signalwaveform; a second bandpass filter connected to receive the output ofsaid half-wave rectifier and having identical characteristic to saidfirst bandpass filter; a second rectifier connected to receive theoutput of said second bandpass filter; and means connected to said firstand second rectifiers to determine the ratio of the output of said firstrectifier to that of said second rectifier.
 24. The apparatus of claim23 wherein said means for generating a test signal waveform includes aramp voltage generator having a potentiometer connected thereacross,said potentiometer comprising a first and a second variable resistors.25. The apparatus of claim 23 wherein said means connected to said firstand second rectifiers includes a pair of logarithmic amplifiers.
 26. Theapparatus of claim 23 further comprising a lowpass filter connected toreceive the output of said half-wave rectifier, and an X-Y recorderhaving an X-axis terminal connected to the output of said lowpass filterand a Y-axis terminal connected to said last-named means connected tosaid first and second rectifiers.
 27. The apparatus of claim 23 whereinsaid circuit forms part of audio equipment such as an audio amplifierand wherein said first and third predetermined time periods are in therange of 0.1 to 5 m sec. and said second predetermined time period is inthe range of 0.05 to 0.5 m sec., the ratio of said first predeterminedtime period to said second predetermined time period being in the rangeof 2 to
 10. 28. The apparatus of claim 23 wherein said circuit formspart of a signal transmission system and wherein said secondpredetermined time period is approximately equal to the reciprocal ofdouble the anticipated maximum frequency of an input signal to saidsignal transmission system.